Polyesters such as poly(ethylene terephthalate) (PET) are engineering thermoplastics used in a wide variety of end use applications such as fibers, films, automotive parts, food and beverage containers and the like. PET can be processed by a variety of techniques including injection molding, compression molding, extrusion, thermoforming, blow molding, and combinations thereof. Extruded into film (a.k.a., sheet) of between 100 and 1000 microns thick, PET may be used as-fabricated or shaped, e.g., by thermoforming, into articles such as displays, signs, credit or debit cards, or packaging articles. For example, extruded PET film material can be used to make trays, packages or containers in which frozen foods can be both stored and heated and/or cooked in an oven. As used herein, the terms tray or trays are intended to include packages and containers in which food, especially frozen food, is packaged and sold for subsequent heating and/or cooking while still in the tray, package or container. Food trays fabricated from crystallized PET retain good dimensional stability over the range of temperatures commonly encountered during both microwave and convection oven cooking.
One problem encountered with PET food trays is that they occasionally break when trays containing frozen food are dropped. One way to improve the low temperature toughness of the trays, as measured by lowering the ductile-to-brittle transition temperature, is to use high molecular weight PET in the fabrication of the tray. Therefore, PET used in food trays often is specially manufactured to an intrinsic viscosity (It.V.) of about 0.90 to about 1.05 dL/g. Another approach is to add an impact modifier to the PET during the film extrusion process. In general, trays are toughest when both approaches are utilized.
U.S. Pat. No. 4,172,859 discloses that polymeric materials that serve well as impact modifiers should (i) possess a modulus 1/10 that of the polyester matrix material, (ii) be well dispersed within the matrix material in discrete phases of 0.01 micron to 3.0 micron in size, and (iii) be well bonded to the matrix.
Low modulus polymers commonly used as impact modifiers fall into several general classes. The first class comprises rubbers based on butadiene or isoprene, e.g., polybutadiene, polyisoprene, natural rubber, styrene-butadiene (SBR), acrylonytrile-butadiene (ABN or nitrile rubber), styrene-butadiene-styrene (SBS) or hydrogenated SBS (styrene-ethylene-butene-styrene) block copolymers (SEBS), or acrylonytrile-butadiene-styrene (ABS) polymers containing high levels of butadiene. Butadiene-based rubbers generally have low glass transition temperatures (Tg's) which help to improve low temperature toughness, but they may not be stable under the high temperatures at which polyesters are processed. The second major class of impact modifiers comprise elastomers based on polyethylene, e.g., ethylene-propylene rubbers (EPR) or EPRs with a small amount of side chain diene moiety (EPDM), ethylene-acrylate copolymers such as ethylene/methyl acrylate, ethylene/ethyl acrylate, ethylene/butyl acrylate and ethylene/methylacrylate/glycidyl methacrylate, or ethylene-vinyl acetate copolymers (EVA). A third group of impact modifiers consists of core-shell impact modifiers such as those that contain a poly(methyl methacrylate) (PMMA) hard shell with either a butadiene methacrylate-butadiene-styrene (MBS) or butyl acrylate (acrylic) core, e.g., PARALOID manufactured by Rohm & Haas Company. Core-shell impact modifiers based on acrylonitrile-butadiene-styrene (ABS) also are commercially available, e.g., BLENDEX manufactured by GE Specialty Chemicals). Other elastomers that may serve as impact modifiers include polyesters, e.g., HYTREL manufactured, by E.I. duPont de Nemours Company and ECDEL manufactured by Eastman Chemical Company, and polyurethanes, e.g., PELLETHANE manufactured by Dow Chemical Company, or silicone rubbers.
By properly matching the melt viscosities of the matrix and impact modifier at the temperatures of melt blending, a fine discrete impact modifier phase can be created by the shearing forces obtained during melt processing. A mixing screw must be designed properly to create the appropriate shear fields during a compounding/extrusion process. However, impact modifiers dispersed by purely mechanical action may re-coalesce during a later stage of processing where shear may be reduced.
Alternatively, impact modifiers can be manufactured to an inherently small size using latex or other polymerization processes. Impact modifiers manufactured in this way often contain a stiff shell of harder polymer and are, therefore, often referred to as core-shell impact modifiers. These impact modifiers can be made in 0.2-0.5 micron sizes ideally suited for impact modification of nylons, polycarbonates and polyesters. Nonetheless, these core-shell impact modifiers also must be dispersed by shearing action during melt processing, and are prone to re-coalesce during later stages of molding or compounding.
One way to enhance dispersion and prevent coalescence is to introduce functional groups into the impact modifier that either are highly soluble in the matrix polymer or will react with the matrix polymer. Interaction between these functional groups and the matrix during compounding creates a thin interlayer of material that makes the impact modifier and matrix more energetically compatible. Compatibility related to these functional groups leads to good mixing and good dispersion of the impact modifier. The enhanced compatibility also will reduce the possibility that the impact modifier phases will re-coalesce later during processing. Therefore, impact modifiers containing functional groups that react rapidly with the matrix polymer produce well-dispersed impact modifier phases of small particle size (See “Rubber Toughened Engineering Plastics”, A. A. Collyer, Chapman & Hall, London, 1994). The incorporation of functional groups into the impact modifier also will ensure a good bond between the impact modifier and matrix, i.e. interfacial adhesion between these immiscible phases.
Impact modifiers can be functionalized with a variety of reactive or non-reactive monomers. These functional monomers can be incorporated into the impact modifier directly during preparation of the impact modifier or subsequently by means of a grafting polymerization step. Non-reactive impact modifiers (for example an EPDM-grafted-SAN) compatibilize themselves to the matrix through a closer matching of solubility parameters, without actually bonding to the matrix polymer. The reactive groups of reactive impact modifiers chemically bond to the matrix polymer but, to be effective, they must do so in the limited time available in the extruder during compounding (i.e., during melt blending).
U.S. Pat. No. 4,172,859 lists a variety of functional groups that may be grafted or copolymerized onto ethylene-based elastomers for use with polyesters and nylons. In practice, maleic anhydride (MAH) functionalized impact modifiers work well for nylons, and there are many commercial products available, e.g., EPR-MAH, EVA-MAH, and SEBS-MAH. However, the reaction between maleic anhydride and polyesters is not fast enough for significant compatibilization in the timescales encountered during normal compounding. A functional group that reacts particularly well with polyesters is the monosubstituted oxirane, or epoxy, group such as is present in glycidyl methacrylate (GMA), glycidyl acrylate, allyl glycidyl ether, and 3,4-epoxy-I-butene (EpB). This patent describes thermoplastic compositions comprising blends of polyesters and epoxy-functionalized random ethylene copolymers.
The following patent documents describe polyester compositions which contain epoxy-containing, ethylene-based polymeric materials (also see Scheirs, J., Additives for the Modification of Poly(Ethylene Terephthalate) to Produce Engineering-Grade Polymers in Modern polyesters, Scheirs, J. and Long, T. E. (Eds), Wiley, New York, 2003, pp. 506-515)). U.S. Pat. No. 4,172,859 describes thermoplastic compositions comprising blends of polyesters and epoxy-functionalized, random ethylene copolymers. This patent makes no reference to catalyst residues present in the polyesters. U.S. Pat. No. 4,284,540 describes the use of ethylene/GMA copolymers as a toughening agent for polyesters when combined with 0.1 to 5 weight percent of an added barium catalyst. This patent notes that PET containing antimony catalyst residues are preferable for promoting a reaction with epoxy-containing olefinic materials. The patent does not, however, provide any data that show any toughness enhancement due to these catalyst residues. U.S. Pat. No. 4,753,980 discloses that polyester compositions containing 3-40 weight percent of either ethylene/ethyl acrylate/GMA terpolymer or ethylene/butyl acrylate/GMA terpolymer possess superior low temperature toughness when compared to analogous polyester compositions which contain an ethylene/methyl acrylate/GMA terpolymer. There is no reference to catalyst residues in this patent. U.S. Pat. No. 6,576,309 discloses a polymeric composition comprising a poly(alkylene terephthalate), 4 wt % to 40 wt % ethylene/methyl acrylate copolymer, and 0.1 wt % to 8 wt % of a compatibilizing olefin/acrylate/GMA terepolymer, however the patent makes no reference to the catalyst residues present. U.S. Pat. Nos. 5,098,953, 5,086,119, 5,086,118, 5,086,116, and 5,068,283 disclose that the toughness of polyester compositions containing ethylene/GMA copolymers or ethylene/alkyl acrylate/GMA terpolymers can be improved by adding a functional crosslinking agent to the compositions. The functional crosslinking agent contains, in one molecule, at least two functional groups having reactivity with epoxy group, carboxyl group or hydroxyl group.
US Appln 20060276587 discloses core-shell impact modifier used in performance polymers (e.g., polyesters) to improve low temperature impact performance. In a preferred embodiment, the shell is polymethylmethacrylate or a copolymer containing at least 85 percent by weight of methylmethacrylate. For modification of polyesters, polyamide or alike, small amounts of reactive functionality are typically incorporated in the shell stage. Such reactive monomer can be glycidyl (meth)acrylate, (meth)acrylic amide, (meth)acrylic acid, maleic anhydride and alike.
U.S. Pat. No. 5,206,291 describes compositions comprising a polyester containing 1,4-cyclohexanedimethanol residues and an ethylene/GMA copolymer. Stewart et al., Polymer Engineering and Science, 33 (11), 675 (1993), discloses that PET containing antimony catalyst residues reacts faster with an ethylene/GMA copolymer than does PET catalyzed by other metals. U.S. Pat. No. 5,436,296 discloses that an ethylene/GMA copolymer may be used to compatibilize blends of polyethylene and polyester. European Patent Publication EP 481,471 131 and Penco et al., Journal of Applied Polymer Science, 57, 329 (1995) disclose compositions comprising a polyester, a linear low density polyethylene, an ethylene/ethyl acrylate/GMA terpolymer and 0.5% to 1% of an amine for the opening of an epoxy ring.
U.S. Pat. Nos. 5,483,001, 5,407,999, and 5,208,292 and Die Angewandte Makromolekulare Chemie, 1992, 196 p 89, disclose polyester compositions having improved toughness which contain an ethylene/alkyl acrylate/GMA terpolymer, an ethylene/alkyl acrylate/maleic anhydride terpolymer, and a catalyst such as dimethylstearylamine which accelerates the reaction between the functional groups of the two terpolymers. U.S. Pat. No. 5,652,306 and European Patent Publication EP 737,715 A2 disclose polyester compositions containing MBS or acrylic-type core-shell impact modifiers combined with small amounts of an ethylene/alkyl acrylate/GMA terpolymer. U.S. Pat. Nos. 7,015,261 and 7,119,152 disclose an improved impact modifier for thermoplastic polyesters. The impact modifier is a blend of (A) a core/shell type impact modifier and (B) a linear copolymer derived from ethylene, (meth)acrylic esters, and monomer containing an epoxy group.
While several of the preceding patents discuss the use of added catalysts to promote a reaction between epoxy-containing ethylene polymers and a polyester, in none of these patents is there any disclosure that the toughness of the blend is affected by the presence of residues of catalysts used in the preparation of the polyester. Polyesters typically are prepared using metal catalysts that remain in the polyester product. Examples of these catalysts include organic and inorganic compounds of arsenic, cobalt, tin, antimony, zinc, titanium, magnesium, gallium, germanium, sodium, lithium and the like. Titanium and antimony compounds are frequently used in the preparation of PET.
There is reference in U.S. Pat. No. 4,284,540 and in Stewart et al. Poly. Eng. & Sci., 33 (11), 675 (1993), that certain residual catalysts present in PET can significantly affect the rate of reaction of epoxy functional polymers with PET. U.S. Pat. No. 4,284,540 notes that, among the above mentioned residual polyester polymerization catalysts, antimony catalyst residues are preferred for promoting the reaction between polyester and an epoxy group. The patent does not, however, provide any toughness data related to the presence of these catalyst residues. Stewart et al. quantified the rate of reaction between PET and a copolymer of ethylene and glycidyl methacrylate (E/GMA) by monitoring the rise in torque of a mixture of these two components in an instrumented mixing bowl (also known as a torque rheometer). According to Stewart et al., torque rheometry provides a simple and straightforward method for monitoring the viscosity of polymer blends as a function of blending time. The rheometer continuously measures the torque required to turn the rotor blades that shear and mix the sample within the mixing bowl. For a given material and set of processing conditions, the torque measured is approximately a linear function of the viscosity of the sample. Any change in viscosity with time is in turn related to such effects as changes in molecular weight of the sample (for example, an increase due to a reaction or a decrease due to degradation) or the formation of grafts, branches or crosslinks in the sample. The work by Stewart et al. shows that mixtures of E/GMA with PET containing antimony catalyst residues gives rise to significant increases in torque with mixing time. This led to the conclusion that PET containing antimony catalyst residues accelerated the reaction between the PET and the E/GMA. PET containing residual antimony catalyst was found to produce a more rapid increase in torque than PET containing other residual catalysts. It is often implied by those knowledgeable in the art that a rapid reaction between the E/GMA and the PET containing residual antimony catalyst should lead to better dispersion of the E/GMA (i.e., a smaller particle size) and better bond to the PET. This superior dispersion should, in turn, lead to improved toughness in the resultant blend. Furthermore, since the PET containing residual antimony catalyst reacts faster than the PET containing other catalysts, it is implied that the use of PET containing antimony catalyst residues should lead to tougher blends with epoxy containing ethylene copolymers.
Contrary to the teachings of the prior art discussed above, the toughness of blends of polyesters with epoxy-containing ethylene polymers is strongly affected by the presence of residues of catalysts used in the manufacture of the polyesters. Indeed, when epoxy-containing ethylene polymers are blended with polyesters containing antimony catalyst residues, the resulting polymer blend exhibits surprisingly low toughness values. Nonetheless, it has been found that superior toughness values are obtained when either (1) a phosphorus compound is added when making blends using polyesters that contain antimony catalyst residues or (2) when making blends using polyesters that do not contain antimony catalyst residues. As disclosed in JP 62-146950 and WO 00/15717 (incorporated herein by reference in its entirety), polyester compositions comprising a polyester containing antimony metal, epoxy-containing impact modifiers, and a phosphorus compound exhibit improved toughness. Further, JP 62-146949 and WO 00/15716 (incorporated herein by reference in its entirety) disclosed toughened polyester compositions comprising epoxy-containing impact modifiers and polyester containing residual tin or titanium and titanium or germanium catalyst, respectively. Also disclosed in WO 00/23520 (incorporated herein by reference in its entirety) are articles manufactured from polymer blends comprising epoxy-containing ethylene polymers and polyesters with residual antimony and phosphorus catalysts and polyesters with residual titanium and/or germanium catalysts.
Not wishing to be bound by any technical theories, it is believed that the antimony catalyst residues present in a polyester accelerates an epoxy-epoxy reaction within the epoxy-containing ethylene copolymer that can proceed concurrently with the reaction between the polyester and the epoxy-containing ethylene copolymer. For example, subsequent work using the same mixing bowl experiment as performed by Stewart et al. has shown that a similar increase in torque with time can be obtained when antimony acetate is added directly into an epoxy-containing ethylene copolymer with no PET present. The resultant epoxy-containing ethylene copolymer is highly crosslinked, suggesting that the catalyst is highly active in promoting reactions within the impact modifier itself. Transmission electron microscope images of blends of antimony-catalyzed PET with epoxy-containing ethylene copolymers show that the impact modifier has formed into large phases, many greater than 1 micron in size. These phases are too large to produce maximum toughness in the PET. It is believed that the phosphorus compound deactivates or partially deactivates the residual antimony catalyst, disabling the epoxy-epoxy reaction and allowing the epoxy-PET reaction.
It has now been discovered that polymer blends comprising certain polyester polymers and certain epoxy-containing impact modifiers exhibit improved toughness when the polyester utilized contain aluminum atoms and alkaline earth atoms or alkali metal atoms or alkali compound, i.e., residues from the use of aluminum atoms and alkaline earth atoms or alkali metal atoms or alkali compound residues in the manufacture of the polyester, especially when combined with phosphorus atoms. Furthermore, it has been found the inventive polymer blends prepared with PET polymers catalyzed with aluminum atoms, alkaline earth atoms or alkali metal atoms or alkali compound residues may be prepared using lower molecular weight PET polymers while maintaining the toughness of polymer blends prepared with conventional PET polymers of much higher molecular weight. Thus the PET polymers of the inventive blends exhibit a melt viscosity closer matched to the polyolefin based impact modifier, facilitating better dispersion of the impact modifier and are less expensive to manufacture. Alternatively, it is possible to obtain a polymer blend containing a polyester polymer and having improved toughness relative to a polymer blend containing a polyester having substantially the same It.V.
U.S. patent application Ser. No. 11/495,431 filed Jul. 28, 2006 and having common assignee herewith, discloses polyester compositions that include aluminum atoms in an amount of at least 3 ppm, based on the weight of the polymer, and that further include alkaline earth metal atoms or alkali metal atoms or alkali compound residues, the polymers having an It.V. of at least 0.72 dL/g obtained through a melt phase polymerization.
U.S. patent application Ser. No. 11/229,238, filed Sep. 16, 2005 and having common assignee herewith, discloses polyester compositions comprising polyester polymers, aluminum atoms, alkaline earth atoms or alkali metal atoms or alkali compound residues, and particles that improve the reheat rate of the compositions.
One aspect of the present invention is based on the catalyst system used in the synthesis of the one or more polyester polymers and the effect the catalyst system has on the toughness of the inventive polymer blends comprising the one or more polyesters polymers and an epoxy-containing impact modifier.
Polyester food trays are conveniently manufactured by first extruding a film of polyester, then thermoforming the tray in a heated die. This thermoforming process both forms the tray and crystallizes the polyester resin. The film material may be prepared in a process separate from the thermoforming process (sometimes referred to as the glass to mold process) or the film material may be prepared in-line with the thermoforming process (sometimes referred to as the melt to mold process). Processes for extruding polyester film and thermoforming the film material to produce crystalline PET (also commonly known as CPET) food trays are well known in the art.